House Bill 1400 allows for abortion after this point in the pregnancy if the woman is at risk of "death or substantial and irreversible physical impairment of a major bodily function" or the fetus has "severe fetal abnormalities," but it includes no exception for rape or incest.

Proponents of this measure argue that they're looking out for the health of the woman, but abortion-rights supporters spoke out immediately against the bill, which will become law on July 1.

"Two years ago, politicians in Mississippi tried to use an underhanded law to close the only abortion clinic in the state, which is now hanging on by a federal court order," Nancy Northup, president and CEO of the Center for Reproductive Rights, said in a statement. "Now, these very same politicians have set their sights on shutting down exceptionally rare instances of abortion care after 20 weeks, of which there were only two in 2012."

Opponents of the law have also argued that the measure actually institutes an 18-week, rather than 20-week, ban because it counts from the woman's last menstrual period rather than from fertilization.

Mississippi currently has only one abortion clinic, and a federal appeals court will hear arguments next week on a state law that requires doctors who perform abortions at the clinic to have admitting privileges at a local hospital.